The record
On September 22, 2016, a user writes that they installed macOS Sierra 10.12 on a 13-inch mid 2012 MacBook Pro the previous day and that Siri gave no response from either its icon or keyboard shortcut. They say changing Siri's language and voice settings did not help. Two days later the poster reported success after following another participant's advice to re-enable the Notification Center process in Terminal and then toggle Siri off and on in System Preferences. Others in the thread had mixed results with the same steps, and this remains an individual account.
Read the original source at Apple Support Community (opens in a new tab) ↗
A documented user report, not an independently reproduced test or an estimate of how often the problem occurs.